The present invention relates to a screen of a continuous digester primarily intended for passing liquor through a layer of pulp. The invention is advantageously employed in removing cooking liquor when producing chemical cellulose pulp or paper pulp in a continuous digester. The invention relates also to a continuous digester having said screen mounted on its wall.
Environmental loading caused by industry is regarded as one of the most serious problems in today's society. In chemical pulp production, great importance has been attached to improving production methods so that environmentally harmful effluents and emissions will be minimized. Special progress has been made in improvements related to bleaching of chemical pulp. New environmentally friendly bleaching methods impose greater requirements than previously on the strength of every fiber being fed into the bleaching. Thus, the digester has to be constructed such that the cooking process will be as gentle for fibers as possible. One way to achieve a gentle cook is to keep the temperature and alkalinity profiles as uniform as possible throughout the whole cross-sectional area of the digester. Achieving a sufficiently uniform cross-section profile imposes great requirements on the screen arrangement, the task of which is to enable an efficient throughput of liquid through a pulp layer.
Recently, it has been noted more and more often that large flows of circulation liquid passing through the screens are needed to achieve the desired uniformity. As will be shown later, large flows like this bring about problems for existing screen types. Hence, it is extremely important to develop new and better screen arrangements.
Existing screens are usually formed by a set of bar screens, which screens are often arranged at a desired location on the wall of the digester in such a way that they form a chessboard-like figure. Such screens are dealt with in the publication WO 9419533, for example.
Each "square" comprises a set of vertically arranged bars. Between the bars there is a slot, through which liquid is inducted. The bars are secured in parallel to a cross member, the screen being provided with an angle bar framework.
A problem with this known screen construction is, for example, that it has a relatively great tendency to clog up as the the chips stick to the slots. This is to a great extent due to the liquid having velocity in the radial direction, outwards through the screen, this velocity being about five times higher than the velocity of chips heading downwards. Typically, the velocity of the liquid is approximately 10-15 mm/s, whereas the corresponding velocity of the chips is approximately 2-3 mm/s. Part of the chips follow the radial liquid flow of higher velocity and stick to the slots between the screen bars. The problem is made even worse by the fact that it is difficult to get the screen bars exactly parallel. If the bottom end of the slot happens to be to some extent narrower than the top end, chips are more likely to stick to the slot. Further, more chips may accumulate upon the portion of chips stuck to the slot, thus clogging up even a larger part of the screen. The greater the clogged-up screen surface is, the more intensely it will continue to clog up further, since the screen surface still unclogged is exposed to greater loading and hereby also to greater risk of clogging up. Due to the location of the screen and the way in which chips are clogged up, cleaning of the screen is very difficult, being thus an action to be avoided as far as possible.
In other words, prior art screens are difficult to run, and according to many, operation of bar screens is the Achilles heel of the so called Kamyr digesters.
As a solution to the problem, Kvaerner Pulping Technologies AB has in its patent SE-B-501243 suggested a slot screen characterized by horizontal screen bars. We have done experiments on the operation according to this solution and found the following two phenomena to make the solution impractical.
Firstly, the chips lie upon each other in the digester, whereby they easily thrust their way into a horizontal slot. The phenomenon is similar to that of throwing a pack of cards into the air; most of the cards, practically speaking all of them, fall on to the floor, only a few possibly remaining standing. The same applies to chips falling on to the digester; they lie upon each other and may thus thrust into a horizontal slot when moving outwards in the radial direction.
Another, even more critical factor is shives. There are always a great number of shives among the chips. They are approximately the size of a match, sometimes bigger, sometimes smaller. The diameter thereof may be 1-3 mm. Part of these shives are found at the walls and screens of the digester. It is easy to verify that the shives rotate downwards along the wall surface, round their longitudinal axis, the axis being in the horizontal plane. In other words, they act like a needle bearing. If there is a horizontal slot in the screen, these shives rotating downwards will end up in this slot having a width of 1-3 mm. In this way, the horizontal slot screen clogs up.
Therefore, the solution to the problem of clogging up cannot be what is suggested in Kvaerner's patent SE 501243, but there is a better, slightly surprising solution. When arranging the slots of the screen inclined, the angle being 30-60 degrees, preferably 45 degrees relative to the horizontal plane, an arrangement is created which eliminates the traditional screen problems without bringing about new problems as does the solution presented in Kvaerner's patent.
Another reason for arranging the slots inclined is that the chips column rotates slowly, due to the scraper at the bottom of the digester. By positioning the inclined screens in the digester and making the surface thereof profiled, a spiral movement is created which pushes the chips column downwards in the digester. The phenomenon is the same as when a screw rotates in a nut.
Characteristics of the screen according to the invention become apparent in the appended claims.